1. Field of the Invention
The present invention relates to a rechargeable battery.
2. Description of Related Art
Rapid reductions in size and weight and addition of multifunction capabilities in consumer mobile telephones, portable electronic devices, mobile information terminals, and the like in recent years have created a need for rechargeable batteries as power sources which satisfy conditions of small size and weight, high energy density, ability to be recharged repeatedly for a long time, and other conditions. Lithium-ion rechargeable batteries, which have high energy density compared to other rechargeable batteries, are the most promising type of rechargeable battery for meeting the needs described above. Various research is under way to develop a superior lithium-ion rechargeable battery.
In order to address global warming and other environmental problems, lithium-ion rechargeable batteries have come to be used in power storage systems that are used in solar power generation systems, wind power generation systems, and the like. As a measure for reducing CO2 and overcoming energy problems, the use of hybrid automobiles (HEV: Hybrid Electric Vehicle) and electric automobiles (EV: Electric Vehicle) having low fuel consumption and low gas emissions is expected to increase, and lithium-ion rechargeable batteries targeted for use as automotive batteries are being developed and brought to market.
Demand for lithium-ion rechargeable batteries is thus increasing not only for mobile devices but for large-scale motive power applications as well. When a lithium-ion rechargeable battery is used in a motive power or electrical power storage system, the battery must be endowed with large capacity to enable discharge over long periods of time, and there is also a need for increased service life.
Lithium-ion rechargeable batteries of various shapes and sizes have been proposed for adaption to these applications. A lithium-ion rechargeable battery is generally formed by arranging a positive electrode having a positive electrode active material layer formed therein and a negative electrode having a negative electrode active material layer formed therein so as to face each other with a separator therebetween, housing the electrode group thus formed in an exterior body (housing container), and then injecting a non-aqueous electrolyte therein. Charging and discharging then occurs by the movement of lithium ions between the positive electrode and the negative electrode.
Known shape types for the electrode group include a coil type in which the electrode group is integrally coiled, and a stacked type in which the positive electrode, the separator, and the negative electrode are stacked in a planar shape. A coiled electrode group is housed in a cylindrical canister (exterior body) to form a cylindrical rechargeable battery (see Japanese Laid-open Patent Publication No. 2000-331656). A stacked electrode group may be covered by a laminate film (exterior body) to form a laminate-type rechargeable battery, or the stacked electrode group may be housed in a prismatic canister to form a prismatic battery.
Such a lithium-ion rechargeable battery generates heat and expands during charging and discharging, and therefore must be heat resistant and pressure resistant. Since an electrolyte solution is also sealed therein, airtightness is required. In the cylindrical rechargeable battery described above, resin is used in a portion of a lid that forms part of the exterior body. The center part of the lid forms the positive electrode terminal and is therefore made of metal, but the periphery thereof insulates from the exterior canister and is therefore formed using a resin gasket. The resin gasket is also subjected to heat and pressure. Resin is more susceptible to heat and pressure than metal and is prone to degrade, and the use thereof therefore leads to reduced durability of the rechargeable battery.
In the laminate rechargeable battery described above, since resin surfaces of laminate films are heat-fused to each other, the resin is made hard and brittle by the heat during fusion, and cannot be considered to have high durability.
In the prismatic battery described above, electrode terminals are laser-welded in through holes in a metal exterior body. Since dissimilar materials are difficult to weld together by laser welding, the materials of the exterior body and the electrode terminals are limited. Furthermore, in materials that include resin, the resin near the weld is melted by laser welding, and a high-strength laminated steel sheet cannot be used as the material for the exterior body.